Method for designing a multiblade radial fan and a multiblade radial fan

ABSTRACT

Specifications of the impeller and the scroll type casing of a multiblade radial fan comprising an impeller having numerous radially directed blades circumferentially spaced from each other and a scroll type casing accommodating the impeller are determined so as to make divergence angle of the scroll type casing substantially coincide with divergence angle of the free vortex formed by the air discharged from the impeller.

TECHNICAL FIELD

The present invention relates to a method for designing a multibladeradial fan and also relates to a multiblade radial fan.

BACKGROUND ART

The radial fan, one type of centrifugal fan, has both its blades andinterblade channels directed radially and is thus simpler than othertypes of centrifugal fans such as the sirocco fan, which hasforward-curved blades, and the turbo fan, which has backward-curvedblades. The radial fan is expected to come into wide use as a componentof various kinds of household appliances.

Quietness of the multiblade radial fan, which has numerous radiallydirected blades disposed at equal circumferential distance from eachother, is heavily affected by the impeller of the multiblade radial fan,compatibility between the impeller and the scroll type casing foraccommodating the impeller, and interference between the tongue of thescroll type casing and the blades of the impeller.

The inventors of the present invention proposed design criteria forenhancing the quietness of the impeller of the multiblade radial fan ininternational application PCT/JP95/00789. No one has ever proposeddesign criteria for achieving compatibility between the impeller and thescroll type casing accommodating the impeller of the multiblade radialfan, or design criteria for decreasing sound caused by interferencebetween the tongue of the scroll type casing and the blades of theimpeller.

DISCLOSURE OF INVENTION

An object of the present invention is to provide design criteria forachieving compatibility between the impeller and the scroll type casingaccommodating the impeller of the multiblade radial fan, therebyenhancing the quietness of the multiblade radial fan.

Another object of the present invention is to provide design criteriafor decreasing sound caused by interference between the tongue of thescroll type casing and the blades of the impeller of the multibladeradial fan, thereby enhancing the quietness of the multiblade radialfan.

Still another object of the present invention is to provide designcriteria for decreasing sound caused by interference between the tongueof the scroll type casing and the blades of the impeller of themultiblade centrifugal fan as generally defined to include themultiblade sirocco fan, the multiblade turbo fan as well as themultiblade radial fan, thereby enhancing the quietness of multibladecentrifugal fans in general.

Another object of the present invention is to provide a method fordriving the impeller of the multiblade radial fan under a systematicallyderived condition of maximum efficiency.

1. Provision of design criteria for achieving compatibility between theimpeller and the scroll type casing accommodating the impeller of themultiblade radial fan, thereby enhancing quietness of the multibladeradial fan.

The inventors of the present invention conducted an extensive study andfound that there is a definite correlation between the flow coefficientof the impeller under the condition of maximum total pressure efficiencyand the specifications of the impeller. The present invention wasaccomplished based on this finding. An aim of the present invention istherefore to determine the specifications of the impeller and the scrolltype casing so as to achieve compatibility between the impeller and thescroll type casing accommodating the impeller under the condition ofmaximum total pressure efficiency of the impeller, thereby decreasingsound caused by incompatibility between the impeller and the scroll typecasing. Moreover, the object of the present invention is to generallydecrease sound caused by incompatibility between the impeller and thescroll type casing.

According to the present invention, there is provided a method fordesigning a multiblade radial fan comprising an impeller having numerousradially directed blades circumferentially spaced from each other and ascroll type casing accommodating the impeller, wherein specification ofthe impeller and the scroll type casing are determined so as to make thedivergence angle of the scroll type casing substantially coincide withthe divergence angle of the free vortex formed by the air dischargedfrom the impeller.

According to the present invention, there is provided a method fordesigning a multiblade radial fan comprising an impeller having numerousradially directed blades circumferentially spaced from each other and ascroll type casing accommodating the impeller, wherein specifications ofthe impeller and the scroll type casing are determined so as to make thedivergence angle of the scroll type casing substantially coincide withdivergence angle of the free vortex formed by the air discharged fromthe impeller under the condition of maximum total pressure efficiency.

According to the present invention, there is provided a multibladeradial fan comprising an impeller having numerous radially directedblades circumferentially spaced from each other and a scroll type casingaccommodating the impeller, wherein specifications of the impeller andthe scroll type casing are determined so as to make divergence angle ofthe scroll type casing substantially coincide with divergence angle ofthe free vortex formed by the air discharged from the impeller.

According to the present invention, there is provided a multibladeradial fan comprising an impeller having numerous radially directedblades circumferentially spaced from each other and a scroll type casingaccommodating the impeller, wherein specifications of the impeller andthe scroll type casing are determined so as to make divergence angle ofthe scroll type casing substantially coincide with divergence angle ofthe free vortex formed by the air discharged from the impeller under thecondition of maximum total pressure efficiency.

It is possible to optimize the quietness of the multiblade radial fan bydetermining the specifications of the impeller and the scroll typecasing so as to make the divergence angle of the scroll type casingsubstantially coincide with the divergence angle of the free vortexformed by the air discharged from the impeller.

It is possible to optimize the quietness of the multiblade radial fan bydetermining the specifications of the impeller and the scroll typecasing so as to make the divergence angle of the scroll type casingsubstantially coincide with the divergence angle of the free vortexformed by the air discharged from the impeller under the condition ofmaximum total pressure efficiency.

According to the present invention, there is provided a method fordesigning a multiblade radial fan, wherein specifications of theimpeller and the scroll type casing are determined so as to satisfy thecorrelation expressed by the formula θ_(z) =tan⁻¹[0.295ε(1-nt/(2πr))(H/H_(t))ξ¹.641 ] (where 0.75≦ε≦1.25, n: number ofradially directed blades, t: thickness of the radially directed blades,r: outside radius of the impeller, H: height of the radially directedblades, H_(t) : height of the scroll type casing, ξ: diameter ratio ofthe impeller, θ_(z) : divergence angle of the scroll type casing).

According to a preferred embodiment of the present invention,specifications of the impeller and the scroll type casing are determinedso as to satisfy the correlation expressed by the formula 3.0°≦θ_(z)≦8.0°.

According to a preferred embodiment of the present invention,specifications of the impeller and the scroll type casing are determinedso as to satisfy the correlation expressed by the formula 0.4≦ξ≦0.8.

According to a preferred embodiment of the present invention,specifications of the impeller and the scroll type casing are determinedso as to satisfy the correlation expressed by the formula H/D₁ ≦0.75(where D₁ : inside diameter of the impeller).

According to a preferred embodiment of the present invention,specifications of the impeller and the scroll type casing are determinedso as to satisfy the correlation expressed by the formula 0.65≦H/H_(t).

According to the present invention, there is provided a multibladeradial fan, wherein specifications of the impeller and the scroll typecasing satisfy the correlation expressed by the formula θ_(z) =tan⁻¹[0.295ε(1-nt/(2πr))(H/H_(t))ξ¹.641 ] (where 0.75≦ε≦1.25, n: number ofradially directed blades, t: thickness of the radially directed blades,r: outside radius of the impeller, H: height of the radially directedblades, H_(t) : height of the scroll type casing, ξ: diameter ratio ofthe impeller, θ_(z) : divergence angle of the scroll type casing).

According to a preferred embodiment of the present invention,specifications of the impeller and the scroll type casing satisfy thecorrelation expressed by the formula 3.0°≦θ_(z) ≦8.0°.

According to a preferred embodiment of the present invention,specifications of the impeller and the scroll type casing satisfy thecorrelation expressed by the formula 0.4≦ξ≦0.8.

According to a preferred embodiment of the present invention,specifications of the impeller and the scroll type casing satisfy thecorrelation expressed by the formula H/D₁ ≦0.75 (where D₁ : insidediameter of the impeller).

According to a preferred embodiment of the present invention,specifications of the impeller and the scroll type casing satisfy thecorrelation expressed by the formula 0.65≦H/H_(t).

When specifications of the impeller and the scroll type casing satisfythe correlation expressed by the formula θ_(z) =tan⁻¹[0.295ε(1-nt/(2πr))(H/H_(t))ξ¹.641 ] (where 0.75≦ε≦1.25, n: number ofradially directed blades, t: thickness of the radially directed blades,r: outside radius of the impeller, H: height of the radially directedblades, H_(t) : height of the scroll type casing, ξ: diameter ratio ofthe impeller, θ_(z) : divergence angle of the scroll type casing),compatibility between the scroll type casing and the impeller isachieved and specific sound level is minimized under the condition ofthe maximum total pressure efficiency of the impeller. Thus, amultiblade radial fan with optimized quietness, wherein sound isminimized under the condition of the maximum efficiency of the impeller,can be designed by determining the specifications of the impeller andthe scroll type casing to satisfy the correlation expressed by the aboveformula.

2. Provision of design criteria for decreasing sound level caused byinterference between the tongue of the scroll type casing and theimpeller of the multiblade radial fan, thereby enhancing quietness ofthe multiblade radial fan, and provision of design criteria fordecreasing sound level caused by interference between the tongue of thescroll type casing and the impeller of the multiblade centrifugal fan asgenerally defined to include the multiblade radial fan, therebyenhancing quietness of multiblade centrifugal fans in general.

Sound caused by interference between the tongue of the scroll typecasing and the blades of the impeller (hereinafter called tongueinterference sound) is, as shown in FIG. 21, caused by the periodicalcollision of the air discharged from the interblade channels of theimpeller and having uneven circumferential velocity distribution withthe tongue of the scroll type casing. Frequency f of the tongueinterference sound is expressed by the formula f=n×z (where n: number ofthe blades of the impeller, z: revolution speed of the impeller).

As shown in FIG. 22, the circumferential velocity distribution of theair discharged from the interblade channels becomes more uniform as thedistance from the impeller increases. It is thought that the manner inwhich the circumferential velocity distribution of the air dischargedfrom the interblade channels becomes uniform varies with thespecifications of the impeller.

The inventors of the present invention conducted an extensive study andfound that there is a definite correlation between the manner in whichthe circumferential velocity distribution of the air discharged from theinterblade channels becomes uniform and the specifications of theimpeller. The present invention was accomplished based on this finding.An object of the present invention is therefore to determine thespecifications of the impeller and the scroll type casing so as to makethe air discharged from the interblade channels collide with the tongueof the scroll type casing after the circumferential velocitydistribution of the air has become fairly uniform, thereby decreasingthe tongue interference sound of the multiblade radial fan, and further,decreasing the tongue interference sound of the multiblade centrifugalfan as generally defined to include the multiblade radial fan.

According to the present invention, there is provided a method fordesigning a multiblade centrifugal fan comprising an impeller havingnumerous blades circumferentially spaced from each other and a scrolltype casing accommodating the impeller, wherein the tongue of the scrolltype casing is located at or outside the radial position where the ratioof the half band width of a jet flow discharged from an interbladechannel to the virtual interblade pitch becomes a certain value near 1.

It is possible to make the air discharged from the interblade channelscollide with the tongue of the scroll type casing after thecircumferential velocity distribution of the air has become fairlyuniform by locating the tongue of the scroll type casing at or outsideof the radial position where the ratio of the half band width of a jetflow discharged from an interblade channel to the virtual interbladepitch becomes a certain value near 1. Thus, the tongue interferencesound of the multiblade centrifugal fan decreases.

According to the present invention, there is provided a method fordesigning a multiblade centrifugal fan comprising an impeller havingnumerous blades circumferentially spaced from each other and a scrolltype casing accommodating the impeller, wherein the tongue of the scrolltype casing is located at or outside the radial position where the ratioof the half band width of a jet flow discharged from an interbladechannel to the virtual interblade pitch at a radial position where thehalf band widths of two adjacent jet flows discharged from two adjacentinterblade channels are equal to the virtual interblade pitch becomes acertain value near 1.

It is possible to make the air discharged from the interblade channelscollide with the tongue of the scroll type casing after thecircumferential velocity distribution of the air has become fairlyuniform by locating the tongue of the scroll type casing at or outsideof the radial position where the ratio of the half band width of a jetflow discharged from an interblade channel to the virtual interbladepitch at a radial position where the half band widths of two adjacentjet flows discharged from two adjacent interblade channels are equal tothe virtual interblade pitch becomes a certain value near 1. Thus,tongue interference sound of the multiblade centrifugal fan decreases.

According to the present invention, there is provided a method fordesigning a multiblade centrifugal fan comprising an impeller havingnumerous blades circumferentially spaced from each other and a scrolltype casing accommodating the impeller, wherein specifications of theimpeller and the scroll type casing are determined so as to satisfy thecorrelation expressed by the formula

A.sub.τ +B<10.0 (where τ=b/δ₃, b=(δ₃ -c)(C₄ /X)+c, c=Cδ₁, δ₁={(2πr)/n}-t, δ₃ =2π(r+X)/n, C_(d) : tongue clearance, n: number of theblades, t: thickness of the blades, r: outside radius of the impeller,A, B, C, X: constants determined through tests).

It is possible to make the air discharged from the interblade channelscollide with the tongue of the scroll type casing after thecircumferential velocity distribution of the air has become fairlyuniform by determining the specifications of the impeller and the scrolltype casing so as to satisfy the correlation expressed by the formula

A.sub.τ +B<10.0 (where τ=b/δ₃, b=(δ₃ -c)(C_(d) /X)+c, c=Cδ₁, δ₁={(2πr)/n}-t, δ₃ =2π(r+X)/n, C_(d) : tongue clearance, n: number of theblades, t: thickness of the blades, r: outside radius of the impeller,A, B, C, X: constants determined through tests). Thus, tongueinterference sound of the multiblade centrifugal fan decreases.

According to the present invention, there is provided a method fordesigning a multiblade centrifugal fan comprising an impeller havingnumerous blades circumferentially spaced from each other and a scrolltype casing accommodating the impeller, wherein specifications of theimpeller and the scroll type casing are determined so as to satisfy thecorrelation expressed by the formula

47.09τ+50.77<10.0 (where τ=b/δ₃, b=(δ₃ -c)(C_(d) /x)+c, X=0.8δ₂,c=0.3δ₁, δ₁ ={(2πr)/n}-t, δ₂ =(2πr)/n, δ₃ =2π(r+X)/n, C_(d) : tongueclearance, n: number of the blades, t: thickness of the blades, r:outside radius of the impeller).

It is possible to make the air discharged from the interblade channelscollide with the tongue of the scroll type casing after thecircumferential velocity distribution of the air has become fairlyuniform by determining the specifications of the impeller and the scrolltype casing so as to satisfy the correlation expressed by the formula

47.09τ+50.77<10.0 (where τ=b/δ₃, b=(δ₃ -c)(C_(d) /X)+c, X=0.8δ₂,c=0.3δ₁, δ₁ ={(2πr)/n}-t, δ₂ =(2πr)/n, δ₃ =2π(r+X)/n, C_(d) : tongueclearance, n: number of the blades, t: thickness of the blades, r:outside radius of the impeller). Thus, the tongue interference sound ofthe multiblade centrifugal fan decreases.

3. Provision of a method for driving the impeller of a multiblade radialfan under a systematically derived condition of maximum efficiency.

The multiblade radial fan is desirably used under the condition ofmaximum efficiency of the impeller. Conventionally the maximumefficiency of the impeller has been achieved by trial and error. Therehas been no method for systematically deriving the condition of maximumefficiency of the impeller. Thus, the conventional multiblade radial fanhas not always been used under the condition of maximum efficiency ofthe impeller.

An object of the present invention is to provide a method for drivingthe impeller of a multiblade radial fan under a systematically derivedcondition of maximum efficiency.

According to the present invention, there is provided a method fordriving the impeller of a multiblade radial fan, wherein the impeller isdriven so as to make the flow coefficient φ equal to0.295ε(1-nt/(2πr))ξ¹.641 (where 0.75≦ε≦1.25, n: number of the radiallydirected blades, t: thickness of the radially directed blades, r:outside radius of the impeller, ξ: diameter ratio of the impeller).

According to a preferred embodiment of the present invention, ξsatisfies the formula 0.4 ≦ξ≦0.8.

The total pressure efficiency of the impeller of the multiblade radialfan becomes maximum when the flow coefficient φ is equal to0.295ξ(1-nt/(2πr))ξ¹.641 (where 0.75≦ξ≦1.25, n: number of the radiallydirected blades, t: thickness of the radially directed blades, r:outside radius of the impeller, ξ: diameter ratio of the impeller).Thus, the impeller of the multiblade radial fan can be driven under thecondition of maximum efficiency by being driven so as to make the flowcoefficient φ equal to 0.295ε(1-nt/(2πr))ξ¹.641.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 is a diagram showing the layout of a measuring apparatus formeasuring air volume flow rate and static pressure of an impeller usedfor measuring the efficiency of the impeller alone.

FIG. 2(a) is a plan view of a tested impeller and

FIG. 2(b) is a sectional view taken along line b--b in FIG. 2(a).

FIG. 3 shows experimentally obtained correlation diagrams between thetotal pressure coefficient of the impeller alone and the flowcoefficient φ.

FIG. 4 shows experimentally obtained correlation diagrams between thetotal pressure coefficient of the impeller alone and the flowcoefficient φ_(x) based on the outlet sectional area of the interbladechannel.

FIG. 5 shows correlation between the diameter ratio ξ of the impellerand the flow coefficient φ_(Xmax) based on the outlet sectional area ofthe interblade channel which gives the maximum total pressure efficiencyof the impeller alone plotted on a log--log graph.

FIG. 6 is an explanatory diagram showing the relation between the flowcoefficient φ and the outlet angle θ of the air discharged from theimpeller.

FIG. 7 shows the configuration of the stream line of the air flowdischarged from the impeller.

FIG. 8 is an explanatory diagram showing the relation between the radialvelocity of the air u at the outlet of the impeller and radial velocityof the air U in the portion of the scroll type casing adjacent to theoutlet of the impeller.

FIG. 9 is a diagram showing the layout of a measuring apparatus formeasuring air volume flow rate and static pressure of a multibladeradial fan.

FIG. 10 is a diagram showing the layout of a measuring apparatus formeasuring the sound pressure level of a multiblade radial fan.

FIG. 11 is a plan view of a tested casing used for measuring the soundpressure level of a multiblade radial fan.

FIG. 12 is a plan view of a tested casing used for measuring the soundpressure level of a multiblade radial fan.

FIG. 13 is a plan view of a tested casing used for measuring the soundpressure level of a multiblade radial fan.

FIG. 14 is a plan view of a tested casing used for measuring the soundpressure level of a multiblade radial fan.

FIG. 15 is a plan view of a tested casing used for measuring the soundpressure level of a multiblade radial fan.

FIG. 16 is a plan view of a tested casing used for measuring the soundpressure level of a multiblade radial fan.

FIG. 17 is a plan view of a tested casing used for measuring the soundpressure level of a multiblade radial fan.

FIG. 18 shows correlation diagram between minimum specific sound levelK_(Smin) and divergence angle of the scroll type casing θ_(z).

FIG. 19 shows correlation diagrams between K=(1-.sub.η(φX)/.sub.η(φXmax)) and φ_(X) /φ_(Xmax).

FIG. 20 shows the air flow in the impeller.

FIG. 21 shows the circumferential velocity distribution of the airdischarged from the interblade channels of the multiblade radial fan.

FIG. 22 shows the manner in which the circumferential velocitydistribution of the air discharged from the interblade channels of themultiblade radial fan becomes uniform.

FIG. 23 shows the velocity distribution of the two-dimensional jet flowdischarged from a nozzle.

FIG. 24 is an explanatory diagram showing the half band width of the airflow discharged from the interblade channel of the multiblade radialfan.

FIG. 25(a) is a plan view of a tested impeller used for measuring thesound pressure level and

FIG. 25(b) is a sectional view taken along line b--b in FIG. 25(a).

FIG. 26 is a plan view of a tested casing used for measuring the soundpressure level of a multiblade radial fan.

FIG. 27 is a plan view of a tested casing used for measuring the soundpressure level of a multiblade radial fan.

FIG. 28 is a plan view of a tested casing used for measuring the soundpressure level of a multiblade radial fan.

FIG. 29 is a plan view of a tested casing used for measuring the soundpressure level of a multiblade radial fan.

FIG. 30 is a plan view of a tested casing used for measuring the soundpressure level of a multiblade radial fan.

FIG. 31 is a plan view of a tested casing used for measuring the soundpressure level of a multiblade radial fan.

FIG. 32 is a plan view of a tested casing used for measuring the soundpressure level of a multiblade radial fan.

FIG. 33 is a plan view of a tested casing used for measuring the soundpressure level of a multiblade radial fan.

FIG. 34 shows an example of the sound level spectrum obtained by thesound pressure level measurement.

FIG. 35 shows the correlation between the nondimensional number π andthe dominant level of the tongue interference sound.

FIG. 36 shows the correlation between (a) the dominant level of thetongue interference sound and (b) the difference between the A-weighted1/3 octave band overall sound pressure level with tongue interferencesound and the A-weighted 1/3 octave band overall sound pressure levelwithout tongue interference sound.

THE BEST MODE FOR CARRYING OUT THE INVENTION

I. Invention relating to the design criteria for achieving compatibilitybetween the impeller and the scroll type casing accommodating theimpeller of the multiblade radial fan.

Preferred embodiments of the present invention will be described.

A. Performance test of the impeller alone

Measurement tests of the total pressure efficiency of the impeller alonewere carried out on multiblade radial fans with different diameterratios.

(1) Test conditions

(a) Measuring apparatus

The measuring apparatus is shown in FIG. 1. An impeller was put in adouble chamber type air volume flow rate measuring apparatus (product ofRika Seiki Co. Ltd., Type F-401). A motor for driving the impeller wasdisposed outside of the the air volume flow rate measuring apparatus.

The air volume flow rate measuring apparatus was provided with abellmouth opposite the impeller. The air volume flow rate measuringapparatus was provided with an air volume flow rate control damper andan auxiliary fan for controlling the static pressure near the impeller.The air flow discharged from the impeller was straightened by astraightening grid.

The air volume flow rate of the impeller was measured using orificeslocated in accordance with the AMCA standard.

The static pressure near the impeller was measured through a staticpressure measuring hole disposed near the impeller.

(b) Tested impellers

The outside diameter and the height of all tested impellers were 100 mmand 24 mm respectively. The thickness of the circular base plate and theannular top plate of all tested impellers was 2 mm. Impellers with fourdifferent inside diameters were made. Different impellers had adifferent number of radially directed flat plate blades disposed atequal circumferential distances from each other and different bladethickness. A total of 8 kinds of impellers were made and tested. Theparticulars of the tested impellers are shown in Table 1, and FIGS. 2(a)2(b).

(2) Measurement, Data processing

(a) Measurement

The air volume flow rate of the air discharged from the impeller and thestatic pressure at the outlet of the impeller were measured for eachother of the 8 kinds of impellers shown in Table 1 when rotated at therevolution speed shown in Table 1, while the air volume flow rate of theair discharged from the impeller was varied using the air volume flowrate control damper.

(b) Data processing

From the measured value of the air volume flow rate of the airdischarged from the impeller and the static pressure at the outlet ofthe impeller, a total pressure efficiency defined by the followingformula was obtained.

η=(P_(s) +P_(v))Q/W

In the above formula,

η: total pressure efficiency

P_(s) : static pressure

P_(v) : (ρ/2)(u² v²): dynamic pressure

ρ: density of the air

u=Q/S: radial velocity of the air at the outlet of the impeller

v=rω: circumferential velocity of the outer periphery of the impeller

S=2πrh: outlet sectional area of the impeller

Q: air volume flow rate of the air discharged from the impeller

W: power

r: outside radius of the impeller

h: height of the blade of the impeller

ω: angular velocity of revolution

(3) Test results

Based on the results of the measurements, a correlation between thetotal pressure efficiency η of the impeller alone and the flowcoefficient of the impeller φ expressed by the following formula wasobtained for each tested impeller. The correlations are shown in FIG.43.

φ=u/v

Based on the results of the measurements, a correlation between thetotal pressure efficiency η of the impeller alone and the flowcoefficient of the impeller φ_(x) based on the outlet sectional area ofthe interblade channel expressed by the following formula was obtainedfor each tested impeller. The correlations are shown in FIG. 4.

φ_(x) =u_(x) /v

In the above formula,

u_(x) =Q/S_(x) : radial air velocity at the outlet of the impeller basedon the outlet sectional area of the interblade channel

S_(x) =(2πr-nt)h: outlet sectional area of the impeller based on theoutlet sectional area of the interblade channel

n: number of the radially directed blades

t: thickness of the radially directed blades

As is clear from FIG. 4, the flow coefficient of the impeller φ_(x)based on the outlet sectional area of the interblade channel which givesthe maximum value of the total pressure efficiency η depends on thediameter ratio of the impeller only and not on the number of the bladesor the breadth of the interblade channel.

Correlation between the diameter ratio of the impeller ξ and the flowcoefficient φ_(Xmax) based on the outlet sectional area of theinterblade channel which gives the maximum value of the total pressureefficiency η was obtained from FIG. 4. FIG. 5 shows the correlationplotted on a log--log graph. As is clear from FIG. 5, the correlationbetween φ_(Xmax) and ξ defines a straight line with the inclination of1.641 on a log--log graph.

As described above, the correlation between φ_(Xmax) and ξ is expressedby the following formula 1.

    φ.sub.Xmax =0.295ξ.sup.1.641                        1

In the above formula,

φ_(Xmax) : flow coefficient based on the outlet sectional area of theinterblade channel which gives the maximum value of the total pressureefficiency η

ξ=D₁ /D: diameter ratio of the impeller

D₁ : inside diameter of the impeller

D: outside diameter of the impeller

φ_(max) corresponding to φ_(Xmax) can be derived from formula 1, thedefinition of φ, i.e. φ=u/v, and the definition of φ_(x), i.e. φ_(x)=u_(x) /v (where u_(x) =Q/S_(x) : radial air velocity at the outlet ofthe impeller based on the outlet sectional area of the interbladechannel, S_(x) =(2πr-nt)h: outlet sectional area of the impeller basedon the outlet sectional area of the interblade channel, n: number of theradially directed blades, t: thickness of the radially directed blades).

φ_(max) is expressed by the following formula 2. ##EQU1## B.Compatibility between the impeller and the scroll type casing (1)Hypothesis

As shown in FIG. 6, flow coefficient φ (φ=u/v) is the tangent of theoutlet angle η of the air discharged from the impeller. It is thoughtthat the air discharged from the impeller forms a free vortex. Thus, asshown in FIG. 7, the crossing angle of concentric circle whose centercoincides with the rotation center of the impeller and the stream lineof the air discharged from the impeller is kept at the outlet angle θ ofthe air discharged from the impeller, i.e. tan⁻¹ φ, irrespective of thedistance from the rotation center of the impeller. Thus, it is thoughtthat compatibility between the scroll type casing and the impeller isachieved and the quietness of the multiblade radial fan is optimizedwhen the divergence angle θ_(z) (logarithmic spiral angle) of the scrolltype casing coincides with tan⁻¹ φ.

Based on the aforementioned results of the measurement test of the totalpressure efficiency of the impeller alone and the aforementioneddiscussion about compatibility between the scroll type casing and theimpeller, it is thought that a multiblade radial fan with optimizedquietness, wherein compatibility between the scroll type casing and theimpeller is achieved and the sound level is minimized when the impelleris driven under the condition of the maximum total pressure efficiency,can be designed by setting the divergence angle θ_(z) of the scroll typecasing at the arctangent of φ_(max) , i.e. tan⁻¹ φ_(max), obtained bythe aforementioned formula 2.

As shown in FIG. 8, the height H of the radially directed blades of theimpeller is different from the height H_(t) of the scroll type casingaccommodating the impeller. Thus, when the radial air velocity at theoutlet of the impeller is u, the radial air velocity U in the portion ofthe scroll type casing for accommodating the impeller adjacent theoutlet of the impeller is U=u(H/H_(t)). Thus, the flow coefficient φ_(s)of the impeller against the scroll type casing is φ_(s) =(H/_(t)) φ(where φ: flow coefficient of impeller alone) and the φ_(Smax) isφ_(Smax) =(H/h_(t)) φ_(max) .

From the above, it is thought that a multiblade radial fan withoptimized quietness wherein compatibility between the scroll type casingand the impeller is achieved and the sound level is minimized when theimpeller is driven under the condition of the maximum total pressureefficiency can be designed by determining the divergence angle θ_(z) ofthe scroll type casing based on the following formula 3. ##EQU2## (2)Confirmation test of compatibility between the scroll type casing andthe impeller

It was confirmed by measurements that the quietness of the multibladeradial fan is optimized when the divergence angle θ_(z) of the scrolltype casing satisfies the formula 3.

(a) Measuring apparatuses

(i) Measuring apparatus for measuring air volume flow rate and staticpressure

The measuring apparatus used for measuring air volume flow rate andstatic pressure is shown in FIG. 9. The fan body of the multibladeradial fan had an impeller, a scroll type casing for accommodating theimpeller and a motor. An inlet nozzle was disposed on the suction sideof the fan body. A double chamber type air volume flow rate measuringapparatus (product of Rika Seiki Co. Ltd., Type F-401) was disposed onthe discharge side of the fan body. The air volume flow rate measuringapparatus was provided with an air volume flow rate control damper andan auxiliary fan for controlling the static pressure at the outlet ofthe fan body. The air flow discharged from the fan body was straightenedby a straightening grid.

The air volume flow rate of the fan body was measured using orificeslocated in accordance with the AMCA standard.

The static pressure at the outlet of the fan body was measured through astatic pressure measuring hole disposed near the outlet of the fan body.

(ii) Measuring apparatus for measuring sound pressure level

The measuring apparatus for measuring sound pressure level is shown inFIG. 10. An inlet nozzle was disposed on the suction side of the fanbody. A static pressure control chamber of a size and shape similar tothose of the air volume flow rate measuring apparatus was disposed onthe discharge side of the fan body. The inside surface of the staticpressure control chamber was covered with sound absorption material. Thestatic pressure control chamber was provided with an air volume flowrate control damper for controlling the static pressure at the outlet ofthe fan body.

The static pressure at the outlet of the fan body was measured through astatic pressure measuring hole located near the outlet of the fan body.The sound pressure level corresponding to a certain level of the staticpressure at the outlet of the fan body was measured.

The motor was installed in a soundproof box lined with sound absorptionmaterial. Thus, the noise generated by the motor was confined.

The measurement of the sound pressure level was carried out in ananechoic room. The A-weighted sound pressure level was measured at apoint on the centerline of the impeller and 1 m above the upper surfaceof the casing.

(b) Test impellers, Tested casings

(i) Tested impellers

No.1 impeller (ξ=0.4). No.4 impeller (ξ=0.58) and No.5 impeller (ξ=0.75)in Table 1 were used as tested impellers.

(ii) Tested casings

The height of the scroll type casing was 27 mm. The divergenceconfiguration of the scroll type casing was a logarithmic spiral definedby the following formula. The divergence angle θ_(z) was 2.5°,3.0°,4.5°, 5.5° and 8.0° for No.1 impeller, 3.5°, 4.1°, 4.5°, 5.5° and8.0° for No.4 impeller and 3.0°, 4.5°, 5.5°, 6.0° and 8.0° for No.5impeller.

r_(z) =r[exp(Φ tanθ₂)]

In the above formula,

r_(z) : radius of the side wall of the casing measured from the centerof the impeller

r: outside radius of the impeller

Φ: angle measured from a base line, 0≦Φ≦2π

φ_(z) : divergence angle of the scroll type casing

The tested casings are shown in FIG. 11 to FIG. 17.

(iii) Revolution speed of the impeller

The revolution speeds of the impeller during the measurement are shownin Table 1.

(c) Measurement

The air volume flow rate of the air discharged from the fan body, thestatic pressure at the outlet of the fan body, and the sound pressurelevel were measured for each of the combination of No.1 impeller(ξ=0.4), No.4 impeller (ξ=0.58), No.5 impeller (ξ=0.75) in Table 1 andthe scroll type casings of FIG. 11 to FIG. 17 when rotated at therevolution speed shown in Table 1, while the air volume flow rate of theair discharged from the fan body was varied using the air volume flowrate control damper.

(d) Data processing

From the measured value of the air volume flow rate of the airdischarged from the fan body, the static pressure at the outlet of thefan body, and the sound pressure level, a specific sound level K_(s)defined by the following formula was obtained.

K_(s) =SPL(A)-10log₁₀ Q(P_(t))²

In the above formula,

SPL(A): A-weighted sound pressure level, dB

Q: air volume flow rate of the air discharged from the fan body, m³ /S

P_(t) : total pressure at the outlet of the fan body, mmAq

(e) Test results

Based on the results of the measurements, a correlation between thespecific sound level K_(s) and the air volume flow rate was obtained foreach combination of No.1 impeller, No.4 impeller and No.5 impeller inTable 1 and the scroll type casings of FIG. 11 to FIG. 17.

The correlation between the specific sound level K_(s) and the airvolume flow rate Q was obtained on the assumption that a correlationwherein the specific sound level K_(s) is K_(s1) when the air volumeflow rate Q is Q₁ exists between the specific sound level K_(s) and theair volume flow rate Q when the air volume flow rate Q and the staticpressure p at the outlet of the fan body obtained by the air volume flowrate and static pressure measurement are Q₁ and p₁ respectively, whilethe specific sound level K_(s) and the static pressure p at the outletof the fan body obtained by the sound pressure level measurement areK_(s1) and p₁ respectively. The above assumption is thought to bereasonable as the size and the shape of the air volume flow ratemeasuring apparatus used in the air volume flow rate and static pressuremeasurement are substantially the same as those of the static pressurecontrolling box used in the sound pressure level measurement.

The measurements showed that the specific sound level K_(s) of eachtested combination of No.1 impeller, No.4 impeller and No.5 impeller inTable 1 and the scroll type casings of FIG. 11 to FIG. 17 varied withthe air volume flow rate or the flow coefficient. The variation of thespecific sound level K_(s) is generated by the effect of the casing.Thus, it can be assumed that the minimum value of the specific soundlevel K_(s), i.e. the minimum specific sound level K_(Smin) in eachcombination of No.1 impeller, No.4 impeller and No.5 impeller in Table 1and the scroll type casings of FIG. 11 to FIG. 17, represents thespecific sound level K_(s) when the outlet angle θ of the air dischargedfrom the impeller against the casing coincides with the divergence angleθ_(z) of the scroll type casing and the impeller becomes compatible withthe scroll type casing.

Correlations between the minimum specific sound level K_(Smin) and thedivergence angle θ_(z) of the scroll type casing are shown in FIG. 18and No.1 impeller, No.4 impeller and No.5 impeller in Table 1.

(F) Discussion

As is clear from FIG. 18, the minimum specific sound level K_(Smin) isminimized when the divergence angle θ_(z) of the scroll type casing is2.5° in No.1 impeller, the minimum specific sound level K_(Smin) isminimized when the divergence angle θ_(z) of the scroll type casing is4.1° in No.4 impeller, and the minimum specific sound level K_(Smin) isminimized when the divergence angle θ_(z) of the scroll type casing in6.0° in No.5 impeller. On the other hand, the optimum value of thedivergence angle θ_(z) of the scroll type casing for No.1 impeller, No.4impeller and No.5 impeller obtained by formula 3 are 2.46°, 3.94° and5.99°, respectively. Thus, the divergence angle of the scroll typecasing which minimizes the minimum specific sound level K_(Smin) is ingood agreement with the optimum divergence angle of the scroll typecasing obtained by formula 3.

The follow facts are clear from the above.

(c) Results of the measurements for No.5 impeller (ξ=0.75) shown in FIG.18 should be observed. The minimum specific sound level K_(Smin) in eachmeasured combination is shown in FIG. 18. As mentioned earlier, theoutlet angle θ of the air discharged from the impeller against thescroll type casing coincides with the divergence angle θ_(z) of thescroll type casing, and the flow coefficient φ_(s) of the impelleragainst the scroll type casing is tanθ_(z) when the specific sound levelK_(s) is K_(Smin). Thus, the flow coefficient φ_(s) of the impelleragainst the scroll type casing is tan3.0° in the measured combination I(the divergence angle θ_(z) of the scroll type casing is θ_(z) =3.0° inthe measured combination I), the flow coefficient φ_(s) of the impelleragainst the scroll type casing is tan4.5° in the measured combination II(the divergence angle θ_(z) of the scroll type casing is θ_(z) =4.5° inthe measured combination II), the flow coefficient φ_(s) of the impelleragainst the scroll type casing is tan5.5° in the measured combinationIII (the divergence angle θ_(z) of the scroll type casing is θ_(z) 325.5° in the measured combination III), the flow coefficient φ_(s) of theimpeller against the scroll type casing in tan6.0° in the measuredcombination IV (the divergence angle θ_(z) of the scroll type casing isθ_(z) =6.0° in the measured combination IV), and the flow coefficientφ_(s) of the impeller against the scroll type casing is tan8.0° in themeasured combination V (the divergence angle θ_(z) of the scroll typecasing is θ_(z) =8.0° in the measured combination V).

Supposing that a multiblade radial fan having No.5 impeller installed inthe scroll type casing with divergence angle of 6.0° is driven underconditions wherein the flow coefficients φ_(s) of the impeller againstthe scroll type casing are tan3.0°, tan4.5°, tan5.5°, tan6.0° andtan8.0°, then the outlet angle θ of the air discharged from the impelleragainst the scroll type casing does not coincide with the divergenceangle θ_(z) (θ_(z) =6.0°) of the scroll type casing under the drivingconditions wherein the flow coefficients φ_(s) of the impeller againstthe scroll type casing are tan3.0°, tan4.5°, tan5.5° and tan8.0°, andthe specific sound levels K_(s) under the driving conditions wherein theflow coefficients φ_(s) of the impeller against the scroll type casingare tan3.0°, tan4.5°, tan5.5° and tan8.0° are larger than the minimumspecific sound levels in the measured combinations I, II, III and Vrespectively, On the other hand, the outlet angle θ of the airdischarged from the impeller against the scroll type casing coincideswith the divergence angle θ_(z) (θ_(z) =6.0°) of the scroll type casingunder the driving condition wherein the flow coefficient φ_(s) of theimpeller against the scroll type casing is tan6.0 °. Thus, the specificsound level K_(s) under the driving condition wherein the flowcoefficient φ_(s) of the impeller against the scroll type casing istan6.0° is equal to the minimum specific sound level in the measuredcombination VI. Thus, the quietness of the multiblade radial fan havingNo.5 impeller installed in the scroll type casing with divergence angleof 6.0° is optimized under the driving condition wherein the the flowcoefficient φ_(s) of the impeller against the scroll type casing istan6.0°.

As mentioned earlier, the optimum value of the divergence angle θ_(z) ofthe scroll type casing against No.5 impeller obtained by the formula 3is 5.99°. The divergence angle θ_(z) obtained by formula 3 is equal tothe arctangent of the flow coefficient φ_(s) of the impeller against thescroll type casing when the impeller is driven under the conditionwherein the total pressure efficiency η is maximum. Thus, the totalpressure efficiency η of No.5 impeller becomes maximum when the flowcoefficient φ_(s) of the impeller against the scroll type casing istan5.99 °.

The above discussion proves for No.5 impeller that a multiblade radialfan wherein the quietness is optimized when the impeller is driven undera condition wherein the total pressure efficiency η is maximum can bedesigned by determining the divergence angle of the scroll type casingbased on formula 3.

In the same way, it is proved for No.1 and No.4 impellers that amultiblade radial fan wherein the quietness is optimized when theimpeller is driven under a condition wherein the total pressureefficiency η is maximum can be designed by determining the divergenceangle of the scroll type casing based on formula 3.

(ii) Results of the measurements for No.5 impeller (ξ=0.75) in FIG. 18should be observed. The minimum specific sound level K_(Smin) in eachmeasured combination is shown in FIG. 18. As is clear from FIG. 18, theminimum specific sound level K_(Smin) is minimized in the measuredcombination IV, that is the minimum specific sound level K_(Smin) isminimized when the divergence angle θ_(z) of the scroll type casing is6.0°. Thus, the quietness of No.5 impeller is optimized when it isinstalled in a casing with divergence angle of 6.0° (it is reasonable toconclude that the minimum specific sound level K_(Smin) is minimized inthe measured combination IV because the total pressure efficiency ofNo.5 impeller becomes maximum, the energy loss of the No.5 impellerbecomes minimum, and the sound of No.5 impeller alone which causes theenergy loss of the No.5 impeller becomes minimum in the measuredcombination IV). On the other hand, the optimum value of the divergenceangle θ_(z) of the scroll type casing against No.5 impeller obtained byformula 3 is 5.99°.

The above discussion proves for No.5 impeller that the quietness of themultiblade radial fan can be optimized by determining the divergenceangle of the scroll type casing based on formula 3.

In the same way, it is proved for No.1 and No.4 impellers that thequietness of the multiblade radial fan can be optimized by determiningthe divergence angle of the scroll type casing based on formula 3.

(3) Design criteria for achieving the compatibility between the impellerand the scroll type casing for accommodating the impeller of themultiblade radial fan.

(a) A multiblade radial fan wherein compatibility between the scrolltype casing and the impeller is achieved, the sound level is minimized,and the quietness is optimized when the impeller is driven under thecondition wherein the total pressure efficiency η is maximum can bedesigned by determining the divergence angle θ_(z) of the scroll typecasing based on formula 3.

(b) The quietness of the multiblade radial fan can be optimized bydetermining the divergence angle θ_(z) of the scroll type casing basedon formula 3.

(c) Further development of the design criteria

(1) Expansion of formula 3

Correlations between K=(1-.sub.η(φx) /.sub.η(φXmax)) and φ_(x) /φ_(Xmax)derived from FIG. 4 are shown in FIG. 19.

As is clear from FIG. 19, the decrease of the total pressure efficiencyη from its maximum value is 6% or so even if φ_(x) is varied ±25% fromφ_(Xmax). As is clear from FIG. 19, the increase of the minimum specificsound level K_(Smin) from its minimum value is 3 dB to 4dB even if φ_(x)is varied ±25% from φ_(Xmax). Thus, it is thought that the efficiencyand the quietness of the multiblade radial fan do not decrease so mucheven if the right side of formula 3 is varied about ±25% when thedivergence angle θ_(z) of the scroll type casing is determining based onformula 3. Thus, it is thought that the following formula 4 can be usedas the design criteria for achieving compatibility between the impellerand the scroll type casing.

    θ.sub.z =tan.sup.-1 [0.295ε(1-nt/(2πr)(H/H.sub.t)ξ.sup.1.641 ]  4

In the above formula, 0.75≦ε≦1.25

(2) Range of the diameter ratio of the impeller

As is clear from FIG. 5, the correlation diagram between the diameterratio ξ of the impeller and the flow coefficient φ_(Xmax) based on theoutlet sectional area of the interblade channel which gives the maximumvalue of the total pressure efficiency η is substantially linear overthe range 0.4≦ξ≦0.9. Judging from this fact, it is thought that formula4 can be expandedly used for an impeller whose diameter ratio ξ is inthe range of 0.3≦ξ≦0.9. However, it is rather hard to achievesatisfactory quietness in an impeller whose diameter ratio ξ is as largeas 0.9 or so, while it is rather hard to dispose numerous radiallydirected blades in an impeller whose diameter ratio ξ is as small as 0.3or so. Thus, formula 4 is preferably used for an impeller whose diameterratio ξ is in the range of 0.4≦ξ≦0.8.

(3) Range of the divergence angle θ_(z) of the scroll type casing

A scroll type casing whose divergence angle θ_(z) is too small cannotprovide a satisfactory air volume flow rate, while a scroll type casingwhose divergence angle θ_(z) is too large is troublesome to handlebecause its outside dimensions are too large. Thus, the divergence angleθ_(z) of the scroll type casing is preferably in the range of 3.0°≦θ_(z)≦8.0°.

(4) Range of H/D₁

When the ratio H/D₁ of the height H of the radially directed blades tothe inside diameter D₁ of the impeller is too large, vortices aregenerated in the interblade channels as shwon in FIG. 20, which degradesthe aerodynamic performance and the quietness of the impeller. Generallyspeaking, the ratio H/D₁ is set at 8.0 to 9.0 in the sirocco fan and 0.6or so in the radial fan. Thus, the the ratio H/D₁ is preferably in therange of H/D₁ ≦0.75.

(5) Rang of H/H_(t)

When the ratio H/H_(t) of the height H of the radially directed bladesto the height of the scroll type casing is to small, the air dischargedfrom the impeller is discharged from the casing before it sufficientlydiffuses in the casing. Thus, some portions of the space in the casingare not effectively utilized. Thus, the ratio H/H_(t) is preferably inthe range of 0.65≦H/H_(t) so as to sufficiently diffuse the airdischarged from the impeller in the casing.

II. Invention to provide design criteria for decreasing sound caused byinterference between the tongue of the scroll type casing and the bladesof the impeller of the multiblade radial fan, and to provide designcriteria for decreasing sound caused by interference between the tongueof the scroll type casing and the blades of the impeller of themultiblade centrifugal fan as generally defined to include themultiblade radial fan

Preferred embodiments of the present invention are described.

A. Theoretical background

L. Prandtl states that the half band width b of a two dimensional jetflow discharged from a nozzle (supposing that the flow velocity of a twodimensional jet flow at its center line L is u_(m), so that half bandwidth b is twice as long as the distance between a point where the flowvelocity u is u=u_(m) /2 and the center line L of the two dimensionaljet flow) is proportional to the distance x from the nozzle shown inFIG. 23 (Prandtl, L., The mechanics of viscous fluids, in W. F. Dureand(ed.): Aerodynamic Theory, III, 16-208(1935)).

The air flow discharged from the interblade channels of the impeller ofthe multiblade radial fan can be regarded as two dimensional jet flowsdischarged from the same number of radially directed nozzles as theblades of the impeller disposed along the outer periphery of theimpeller.

Supposing that, as shown in FIG. 24, the breadth of the interbladechannel at the outer periphery of the impeller of the multiblade radialfan is δ₁, the interblade pitch at the outer periphery of the impellerof the multiblade radial fan is δ₂, the half band width of the air flowdischarged from the interblade channel at the outer periphery of theimpeller of the multiblade radial fan is c, the radial distance of thepoint where the half band width of the air flow discharged from theinterblade channel is equal to the virtual interblade pitch (supposingthat the blades extend radially beyond the outer periphery of theimpeller, so that the virtual interblade pitch is the interblade pitchin the region where the blades extend radially beyond the outerperiphery of the impeller) from the outer periphery of the impeller isX, the virtual interblade pitch at the point where the radial distancefrom the outer periphery of the impeller is X is δ₃, and the distancefrom the outer periphery of the impeller is x, then the half band widthb of the air flow discharged from the interblade channel of the impellerof the multiblade radial fan is obtained by the following formula basedon the theory of Prandtl.

    b=(δ.sub.3 -c)x/X+c                                  5

δ₁, δ₂ and δ₃ are obtained by the following formulas.

    δ.sub.1 ={(2πr)/n}-t                              6

    δ.sub.2 =(2πr)/n                                  7

    δ.sub.3 =2π(r+X)/n                                8

In the above formulas, n is number of the blades, t is thickness of theblades, and r is outside radius of the impeller.

b is divided by δ₃ so as to make the formula 5 nondimensional. Then,##EQU3##

It can be conclude that the nondimensional number τ represents thedegree of the diffusion of the air flow discharged from the interbladechannel of the impeller of the multiblade radial fan, or the degree ofthe uniformization of the circumferential distribution of the airvelocity. Thus, it is thought that the design criteria for decreasingthe tongue interference sound of the multiblade radial fan can beobtained by using the nondimensional number τ.

B. Sound level measurement tests

Sound level measurement tests were carried out on a plurality ofimpellers of the multiblade radial fan with different diameter ratio.

(1) Test conditions

(a) Tested impellers, Tested casings

(i) Tested impellers

A total of 39 kinds of impellers with different outside diameter,diameter ratio, number of blades, blade thickness, etc. were made andtested.

The particulars of the tested impellers are shown in Table 2, and FIGS.25(a) and 25(b).

(ii) Tested casings

The height of the scroll type casings was 27 mm. The divergenceconfiguration of the scroll type casings was a logarithmic spiraldefined by the following formula. The divergence angle θ₂ was 4.50°.

    r.sub.z =r[exp(θ tan θ.sub.z)]

In the above formula,

r_(z) : radius of the side wall of the casing measured from the centerof the impeller

r: outside radius of the impeller

θ: angle measured from a base line, 0≦θ≦2π

θ_(z) : divergence angle of the scroll type casing

A plurality of casings with different tongue radius R and tongueclearance C_(d) were made for each group of impellers with the sameoutside diameter so as to accommodate the impellers belonging to thegroup and were tested. The tested casings are shown in FIGS. 26 to 33.

(b) Measuring apparatuses

(i) Measuring apparatus for measuring air volume flow rate and staticpressure

The measuring apparatus used for measuring air volume flow rate andstatic pressure is shown in FIG. 9. The fan body had an impeller, ascroll type casing for accommodating the impeller and a motor. An inletnozzle was disposed on the suction side of the fan body. A doublechamber type air volume flow rate measuring apparatus (product of RikaSeiki Co. Ltd., Type F-401) was disposed on the discharge side of thefan body. The air volume flow rate measuring apparatus was provided withan air volume flow rate control damper and an auxiliary fan forcontrolling the static pressure at the outlet of the fan body. The airflow discharged from the fan body was straightened by a straighteninggrid.

The air volume flow rate of of the air discharged from the fan body wasmeasured using orifices located in accordance with the AMCA standard.The static pressure at the outlet of the fan body was measured through astatic pressure measuring hole disposed near the outlet of the fan body.

(ii) Measuring apparatus for measuring sound pressure level

The measuring apparatus for measuring sound pressure level is shown inFIG. 10. An inlet nozzle was disposed on the suction side of the fanbody. A static pressure control chamber of a size and shape similar tothose of the air volume flow rate measuring apparatus was disposed onthe discharge side of the fan body. The inside surface of the staticpressure control chamber was covered with sound absorption material. Thestatic pressure control chamber was provided with an air volume flowrate control damper for controlling the static pressure at the outlet ofthe fan body.

The static pressure at the outlet of the fan body was measured through astatic pressure measuring hole located near the outlet of the fan body.The sound pressure level corresponding to a certain level of the staticpressure at the outlet of the fan body was measured.

The motor was installed in a soundproof box lined with sound absorptionmaterial. Thus, the noise generated by the motor was confined.

The measurement of the sound pressure level was carried out in ananechoic room. The A-weighted sound pressure level was measured at apoint on the centerline of the impeller and 1 m above the upper surfaceof the casing.

(2) Measurement

Measurements were carried out as follows.

(a) One specific impeller belonging to one specific group of impellerswith the same outside diameter, number of blades and blade thickness wasset in one specific casing belonging to the corresponding group ofcasings with different tongue radius and tongue clearance.

(b) Sound level of the fan was measured for each of a plurality ofcombinations of the air volume flow rate of the discharged air from thefan and the revolution speed of the impeller with the same flowcoefficient φ of 0.106.

The reason for setting the flow coefficient φ at 0.106 will beexplained.

As shown in FIG. 6, flow coefficient φ (φ=u/v, u=Q/S: radial airvelocity at the outlet of the impeller, v=rω: circumferential velocityof the impeller of the outer periphery of the impeller, Q: air volumeflow rate, S=2πrh: outlet sectional area of the impeller, r: outsideradius of the impeller, h: height of the impeller, ω: angular velocityof the impeller) is the tangent of the outlet angle θ of the airdischarged from the impeller. It is thought that the air discharged fromthe impeller forms a free vortex. Thus, as shown in FIG. 7, the crossingangle of a concentric circle whose center coincides with the rotationcenter of the impeller and the stream line of the air discharged fromthe impeller is kept at the outlet angle θ of the air discharged fromthe impeller, i.e. tan⁻¹ φ, irrespective of the distance from therotation center of the impeller. Thus, compatibility between the scrolltype casing and the impeller is achieved and the sound caused byincompatibility between the scroll type casing and the impeller iseliminated when the divergence angle θ_(z) (logarithmic spiral angle) ofthe scroll type casing coincides with tan⁻¹ φ. In the presentmeasurement, tan⁻¹ φ was made coincide with the divergence angle θ_(z)of the scroll type casing, i.e. 4.5°, so as to eliminate sounds otherthan the tongue interference sound as far as possible. Thus, the flowcoefficient φ was set at 0.106.

The correlation between the sound level of the fan and the air volumeflow rate of the discharged air from the fan was obtained on theassumption that a correlation wherein the specific sound level is K₁when the air volume flow rate is Q₁ exists between the specific soundlevel K and the air volume flow rate Q when the air volume flow rate andthe static pressure at the outlet of the fan body obtained by the airvolume flow rate and specific pressure measurement are Q₁ and p₁respectively, while the specific sound level and the static pressure atthe outlet of the fan body obtained by the sound pressure levelmeasurement are K₁ and p₁ respectively. The above assumption is thoughtto be reasonable as the size and the shape of the air volume flow ratemeasuring apparatus used in the air volume flow rate and static pressuremeasurement are substantially the same as those of the static pressurecontrolling box used in the sound pressure level measurement.

(c) Dominant level of the tongue interference sound was obtained byvisually inspecting the spectrum of the measured sound for each of theplurality of combinations of air volume flow rate of the discharged airfrom the fan and the rotation velocity of the impeller with the samevalue, 0.106, of the flow coefficient φ. The dominant level of thetongue interference sound was obtained as the difference between thetongue interference sound level and the mean value of the sound level inthe frequency range near the frequency of the tongue interference sound.The dominant level of the tongue interference sound of the specific oneimpeller set out in (a) was obtained as the mean value of the pluralityof dominant levels of the tongue interference sound obtained by theaforementioned procedure. One example of the spectra obtained by thesound level measurements is shown in FIG. 34. One example of the resultsof the sound level measurements for one specific impeller is shown inTable 3.

(d) Another one specific impeller belonging to the one specific group ofthe impellers set out in (a) was set in the one specific casing set outin (a) so as to carry out (b) and (c), thereby obtaining the dominantlevel of the tongue interference sound of the another one specificimpeller. In the same way, the dominant levels of the tongueinterference sound of all of the impellers belonging to the one specificgorup set out in (a) were obtained.

(e) The dominant level of the tongue interference sound of thecombination of the one specific group of the impellers set out in (a)and the one specific casing set out in (a) was obtained as the meanvalue of a plurality of dominant levels of the tongue interference soundobtained by (c) and (d). One specific test was defined by a series ofthe procedures (a) to (e).

(f) In the same way as (a) to (e), the dominant level of the tongueinterference sound of the combination of the one specific group of theimpellers set out in (a) and another one specific casing belonging tothe group of the casings set out in (a) was obtained. Another onespecific test was defined by a series of the procedures of (f).

(g) In the same way as (f), a total of 47 kinds of tests were carriedout for a total of 47 kinds of combinations of a plurality of groups ofthe impellers and a plurality of casings so as to obtain dominant levelsof the tongue interference sound.

Test results are shown in Table 4. In Table 4, impeller numbersbelonging to the group of the impellers, casing number, specificationsof the impellers, specifications of the casing and the dominant level ofthe tongue interference sound corresponding to each test are also shown.

(3) Discussion

(a) Correlation between the tongue interference sound and thenondimensional number τ

It is though that, if the half band width b of the air flow dischargedfrom the interblade channel is equal to or larger than δ₃ at the radialposition of the tongue of the scroll type casing in FIG. 24, then thetongue interference sound is hardly generated because the velocitydistribution of the air flow discharged from the interblade channel isfairy uniform at the radial position of the tongue of the scroll typecasing. That is, it is thought that, if τ obtained by formula 9 is equalto or larger than 1 when tongue clearance C_(d) of the scroll typecasing is substituted for x in formula 5, then the tongue interferencesound is hardly generated.

It is supposed that, also in Table 4, τ of each combination of the groupof the impellers and the scroll type casing corresponding to the testnumber wherein the tongue interference sound did not appear, obtained bysubstituting the tongue clearance C_(d) of the scroll type casing of theaforementioned combination for x in formula 5, calculating formulas 6 to8 using the outside radius r, number of blades n, and blade thickness tof the group of the impellers of the aforementioned combination, andcalculating τ based on formula 9, is equal to or greater than 1.

Based on the aforementioned supposition, τ was obtained for each testnumber in Table 4 by substituting the tongue clearance C_(d) of thecorresponding scroll type casing for x in formula 5, calculatingformulas 6 to 8 using the outside radius r, number of blades n, andblade thickness t of the corresponding group of the impellers, andcalculating τ based on formula 9. Thereafter, X and c in formula 5 wasdetermined so as to make the threshold value of τ (if τ is smaller thanthe "threshold value", then the tongue interference sound does notappear, i.e. the dominant level of the tongue interference sound becomesnegative, while if τ is equal to or larger than the "threshold value",then the tongue interference sound appears, i.e. the dominant level ofthe tongue interference sound becomes positive) is substantially equalto 1. The determined value of X and c are as follows.

    X=0.8δ.sub.2, c=0.3δ.sub.1

τ was obtained for each test number in Table 4 by substituting thetongue clearance C_(d) of the corresponding scroll type casing for x informula 5, substituting 0.8δ₂ and 0.3δ₁ for X and c i formula 5respectively, calculating formulas 6 to 8 using the outside radius r,number of blades n, and blade thickness t of the corresponding group ofthe impellers, and calculating τ based on formula 9. The calculatedvalues of τ are shown in Table 4.

Correlations between τ in Table 4 and the dominant level of the tongueinterference sound are shown in FIG. 35. As is clear from FIG. 35, inspite of some degree of scattering, there is a definite correlationbetween τ in Table 4 and the dominant level of the tongue interferencesound wherein the dominant level of the tongue interference sound issubstantially zero in the region of τ equal to or larger than 1 andlinearly increases as τ decreases in the region of τ smaller 1. Asmentioned earlier, the dominant levels of the tongue interference soundshown in Table 4 are mean values of the results of the numerous soundlevel measurements. So, it is thought that measurement errors are small.Thus, the correlation of FIG. 35 is sufficiently trustworthy.

The correlation between τ and the dominant level of the tongueinterference sound in the region of τ smaller than 1 in FIG. 35 can beapproximated to the following line by the least square approximationmethod.

    Z=-47.09τ+50.77

In the formula, Z is the dominant level of the tongue interferencesound.

(b) Allowable value of the dominant level of the tongue interferencesound

Generally, the A-weighted (0 to 20 kH_(z)), 1/3 octave band overallsound pressure level is used in sound pressure level measurement.Considering the characteristics of the A-weighted filter, sound pressurelevel measurements wherein tongue interference sound with a frequencyrange of about 2 KH_(z) to 7 KH_(z) appeared were observed for aplurality of impellers. In the observed measurements, the A-weighted,1/3 octave band overall sound pressure level was compared with theA-weighted, 1/3 octave band overall sound pressure level without the 1/3octave band sound pressure level of the frequency range wherein thetongue interference sound was present.

The results of the comparison are shown in Table 5. Dominant levels ofthe tongue interference sound derived from the spectra of the sound arealso shown in Table 5. Correlations between the dominant level of thetongue interference sound and the different between the 1/3 octave bandoverall sound pressure level with the tongue interference sound and the1/3 octave band overall sound pressure level without the tongueinterference sound are shown in FIG. 36.

As is clear from Table 5 and FIG. 36, when the dominant level of thetongue interference sound is equal to or less than 10 dB, the differencebetween the 1/3 octave band overall sound pressure level with the tongueinterference sound and the 1/3 octave band overall sound pressure levelwithout the tongue interference sound is equal to or less than 0.5 dB.Considering the fact that the allowable value of measurement error of aprecision sound level meter is 0.5 dB, the difference of 0.5 dB is notsignificant for A-weighted, 1/3 octave band overall sound level. Thus,it is thought that, if the dominant level of the tongue interferencesound is restricted equal to 10 dB or less, the tongue interferencesound does not sound noisy to a person. Actually, the tongueinterference sound with a dominant level equal to or less than 10 dB wasnot considered noisy by those making the measurement.

Thus, it is thought that the tongue interference sound can besufficiently decreased by setting the allowable value of the dominantlevel of the tongue interference sound at 10 dB.

C. Design criteria

The following design criteria for decreasing the tongue interferencesound of the multiblade radial fan are derived from the aforementioneddiscussion.

The specifications of the impeller and the scroll type casing should bedetermined to satisfy the following formula.

    -47.09τ+50.77<10.0 (where τ=b/δ.sub.3,

    b=(δ.sub.3 -c)(C.sub.d /X)+c, X=0.8δ.sub.2, c=0.3 δ.sub.1,

    δ.sub.1 ={(2πr)/n}-t, δ.sub.2 =(2πr)/n,

δ₃ =2π(r+X)/n, C_(d) : tongue clearance, n: number of the blades, t:thickness of the blades, r: outside radius of the impeller).

An embodiment of the present invention regarding the design criteria fordecreasing the sound caused by the interference between the tongue ofthe scroll type casing and the impeller has been described above.However, the present invention is not restricted to the above describedembodiment.

The above described embodiment concerns the multiblade radial fan havingan impeller with numerous radially directed blades disposed at an equalcircumferential distance from each other and a scroll type casing foraccommodating the impeller. However, it is though that the same designcriteria as for the multiblade radial fan can be obtained for themultiblade centrifugal fan wherein the leading edges of the blades ofthe multiblade radial fan are knuckled or bent in the direction ofrotation (if the leading edges of the radially directed blades are bentin the direction of rotation, inlet angle of the air into the interbladechannels decreases, and the sound level decreases), the multibladesirocco fan having an impeller with numerous forward-curved bladesdisposed at an equal circumferential distance from each other and ascroll type casing for accommodating the impeller, the multiblade turbofan having an impeller with numerous backward-curved blades disposed atan equal circumferential distance from each other and a scroll typecasing for accommodating the impeller, etc., by carrying out the samesound level measurements as described above, determining X and c informula 5, obtaining the same correlations between τ and the dominantlevel of the tongue interference sound as shown in FIG. 35, anddeterming the same correlation line as shown in FIG. 35.

As is clear from FIG. 35, the relation -47.09τ+50.77<10.0 is equivalentto the relation τ<0.866. Thus, the aforementioned design criteria areequivalent to the design rule "the tongue of the scroll type casingshould be located at or outside of the radial position where the ratioof the half band width of a jet flow discharged from an interbladechannel to the virtual interblade pitch at a radial position where thehalf band width of adjacent two jet flows discharged from adjacent twointerblade channels are equal to the virtual interblade pitch is 0.866."It is though that the aforementioned ratio varies with the type of thecentrifugal fan and can be determined based on the sound levelmeasurement. Thus, it is thought that the tongue interference sound ofthe multiblade centrifugal fan can be generally decreased by "locatingthe tongue of the scroll type casing at or outside of the radialposition where the ratio the half band width of a jet flow dischargedfrom an interblade channel to the virtual interblade pitch at a radialposition where the half band width of the adjacent two jet flowsdischarged from adjacent two interblade channels are equal to thevirtual interblade pitch is a certain value near 1".

It is though that the half band width of a jet flow discharged from aninterblade channel increases as the distance from the outer periphery ofthe impeller increases, and the ratio of the half band width of a jetflow at a certain radial position to the virtual interblade pitch at theradial position increases as the distance from the outer periphery ofthe impeller increases. Thus, it is thought that it is possible to makethe air discharged from the interblade channels collide with the tongueof the scroll type casing after the circumferential velocitydistribution of the air has become fairly uniform so as to decrease thetongue interference sound of the multiblade centrifugal fan by "locatingthe tongue of the scroll type casing at or outside of the radialposition where the ratio of the half band width of a jet flow dischargedfrom an interblade channel to the virtual interblade pitch is a certainvalue near 1."

III Invention of a method for driving the impeller of the multibladeradial fan under a systematically derived condition of maximumefficiency

As is clear from the aforementioned formula 2, the impeller of themultiblade radial fan can be driven under the condition of maximumefficiency by driving the impeller so as to make the flow coefficient φequal to 0.295(1-nt/2πr))ξ¹.641 (where n: number of the radiallydirected blades, t: thickness of the radially directed blades, r:outside radius of the impeller, ξ: diameter ratio of the impeller).

As pointed out earlier, it is clear from FIG. 19 that the decrease ofthe total pressure efficiency η from its maximum value is 6% or so evenif φ_(x) is varied ±25% from φ_(Xmax). Thus, it is thought that, whenthe driving condition of the multiblade radial fan is determined basedon formula 2, the efficiency of the multiblade radial fan does notdecrease so much even if the right side of formula 2 is varied about±25%. Thus, it is thought that the following formula 10 can be used asthe design criteria for systematically determining the driving conditionof the maximum efficiency of the impeller of the multiblade radial fan.

    φ=0.295ε(1-nt/(2πr)) ξ.sup.1.641         10

In the above formula, 0.75≦ε≦1.25

As is clear from FIG. 5, the correlation diagram between the diameterratio ξ of the impeller and the flow coefficient φ_(Xmax) based on theoutlet sectional area of the interblade channel which gives the maximumvalue of the total pressure efficiency is substantially linear over therange 0.4≦ξ≦0.9. Judging from this fact, it is thought that formula 10can be expandedly used for an impeller whose diameter ratio ξ is in therange of 0.3≦ξ≦0.9. However, it is rather hard to achieve thesatisfactory quietness in an impeller whose diameter ratio ξ is as largeas 0.9 or so, while it is rather hard to dispose numerous radiallydirected blades in an impeller whose diameter ratio ξ is as small as 0.3or so. Thus, formula 10 is preferably used for an impeller whosediameter ratio ξ is in the range of 0.4≦ξ≦0.8.

Load on the impeller of the multiblade radial fan varies and the drivingcondition of the impeller of the multiblade radial fan varies with theshape and the size of the casing for accommodating the impeller of themultiblade radial fan and the nozzle and duct connected to the casing.Thus, the shape and the size of the casing for accommodating theimpeller of the multiblade radial fan and the nozzle and duct connectedto the casing should be adequately studied so as to realize the drivingcondition determined by formula 10.

INDUSTRIAL APPLICABILITY OF THE INVENTION

A multiblade radial fan and a multiblade centrifugal fan with optimizedquietness can be obtained by applying the design criteria in accordancewith the present invention.

The multiblade radial fan can be driven under the condition of themaximum efficiency by applying the design criteria in accordance withthe present invention.

                                      TABLE 1                                     __________________________________________________________________________                                  Rotation speed of                                    Outside                                                                            Inside              the impeller at                                                                        Rotation speed of                           diameter                                                                           diameter            the measurement                                                                        the impeller at                             of the                                                                             of the    Number                                                                             Blade                                                                              of the efficiency                                                                      the measurement                        Impeller                                                                           impeller                                                                           impeller                                                                           Diameter                                                                           of   thickness                                                                          of the impeller                                                                        of the sound level                     No.  (mm) (mm) ratio                                                                              blades                                                                             (mm) alone (rpm)                                                                            (rpm)                                  __________________________________________________________________________    1    100  40   0.40 120  0.3  5400     see note 1                             2    100  40   0.40 40   0.5  5400                                            3    100  58   0.58 144  0.3  5400                                            4    100  58   0.58 144  0.5  5400     7000                                   5    100  75   0.75 144  0.5  5400     see note 2                             6    100  75   0.75 100  0.5  5400                                            7    100  90   0.90 240  0.5  5400                                            8    100  90   0.90 120  0.5  5400                                            __________________________________________________________________________     note 1:                                                                       5000, but 7000 for θ.sub.z = 2.5                                        note 2:                                                                       5000, but 7000 for θ.sub.2 = 4.5°, 5.5°, 6.0         

                                      TABLE 2                                     __________________________________________________________________________                                      Ratio                                                                              Inlet Outlet                                                             of the                                                                             breadth                                                                             breadth                                                            height                                                                             of the                                                                              of the                                Outside                                                                            Indise    Number                                                                             Blade                                                                              Blade                                                                             to the                                                                             interblade                                                                          interblade                       Impeller                                                                           diameter                                                                           diamter                                                                            Diameter                                                                           of   thickness                                                                          height                                                                            Outside                                                                            channel                                                                             channel                          No.  (mm) ((mm)                                                                              ratio                                                                              blades                                                                             (mm) (mm)                                                                              diameter                                                                           (mm)  (mm)                             __________________________________________________________________________    1    99.0 58.0 0.59 120  0.50 20.0                                                                              0.20 1.02  2.09                             2    99.0 40.0 0.40 100  0.50 20.0                                                                              0.20 0.76  2.61                             3    99.0 58.0 0.59 100  0.50 20.0                                                                              0.20 1.32  2.61                             4    99.0 75.0 0.76 100  0.50 20.0                                                                              0.20 1.86  2.61                             5    99.0 90.0 0.91 100  0.50 20.0                                                                              0.20 2.33  2.61                             6    99.0 75.0 0.76 100  0.50 20.0                                                                              0.20 5.39  7.28                             7    99.0 75.0 0.76 60   0.50 20.0                                                                              0.20 3.43  4.68                             8    99.0 75.0 0.76 80   0.50 20.0                                                                              0.20 2.45  3.39                             9    99.0 75.0 0.76 120  0.50 20.0                                                                              0.20 1.46  2.09                             10   99.0 75.0 0.76 144  0.50 20.0                                                                              0.20 1.14  1.66                             11   99.0 58.0 0.59 40   0.50 20.0                                                                              0.20 4.06  7.28                             12   99.0 58.0 0.59 60   0.50 20.0                                                                              0.20 2.54  4.68                             13   99.0 58.0 0.59 80   0.50 20.0                                                                              0.20 1.78  3.39                             14   99.0 90.0 0.91 120  0.50 20.0                                                                              0.2D 1.86  2.09                             15   99.0 58.0 0.59 144  0.50 20.0                                                                              0.20 0.77  1.66                             16   99.0 58.0 0.59 120  0.30 20.0                                                                              0.20 1.22  2.29                             17   99.0 58.0 0.59 144  0.30 20.0                                                                              0.20 0.97  1.86                             18   99.0 58.0 0.59 180  0.30 20.0                                                                              0.20 0.71  1.43                             19   99.0 75.0 0.76 300  0.30 20.0                                                                              0.20 0.49  0.74                             20   99.0 58.0 0.59 10   0.50 20.0                                                                              0.20 17.72 30.60                            21   99.0 40.0 0.40 40   0.50 20.0                                                                              0.20 2.64  7.28                             22   99.0 58.0 0.59 60   1.00 20.0                                                                              0.20 2.04  4.18                             23   99.0 58.0 0.59 30   2.00 20.0                                                                              0.20 4.07  8.37                             24   99.0 90.0 0.91 240  0.50 20.0                                                                              0.20 0.68  0.80                             25   99.0 40.0 0.40 120  0.30 20.0                                                                              0.20 0.75  2.29                             26   100.0                                                                              58.0 0.58 60   0.30 20.0                                                                              0.20 2.74  4.94                             27   100.0                                                                              58.0 0.58 80   0.30 20.0                                                                              0.20 1.98  3.63                             28   100.0                                                                              58.0 0.58 100  0.30 20.0                                                                              0.20 1.52  2.84                             29   100.0                                                                              58.0 0.58 120  0.50 60.0                                                                              0.60 1.02  2.12                             30   100.0                                                                              58.0 0.58 120  0.50 60.0                                                                              0.60 1.02  2.12                             31   70.0 40.6 0.55 90   0.50 28.0                                                                              0.40 0.92  1.94                             32   70.0 52.5 0.75 90   0.50 28.0                                                                              0.40 1.33  1.94                             33   150.0                                                                              87.0 0.58 200  0.50 30.0                                                                              0.20 0.87  1.86                             34   150.0                                                                              112.5                                                                              0.75 200  0.50 30.0                                                                              0.20 1.27  1.86                             35   70.0 40.6 0.58 100  0.30 28.0                                                                              0.40 0.95  1.90                             36   70.0 40.6 0.58 120  0.30 28.0                                                                              0.40 0.76  1.53                             37   150.0                                                                              87.0 0.58 200  0.50 65.0                                                                              0.43 0.87  1.86                             35   100.0                                                                              58.0 0.58 240  0.30 20.0                                                                              0.20 0.46  1.01                             39   100.0                                                                              58.0 0.58 200  0.30 20.0                                                                              0.20 0.61  1.27                             __________________________________________________________________________

                                      TABLE 3                                     __________________________________________________________________________    Impeller No. 23 (mean value of the dominant level of the tongue               interference sound = 24.63 dB)                                                Divergence angle of the scroll type casing θ.sub.z = 4.5°,       Tongue clearance = 3.5 mm                                                     Tongue R = 4.0 mm                                                                                         Frequency of the                                                                       Dominant level of                               Flow        Rotation speed of                                                                      tongue   the tongue                               Measurement                                                                          coefficient                                                                         Number of                                                                           the impeller                                                                           interference soud                                                                      interference sound                       No.    φ blades                                                                              (rpm)    (H.sub.z)                                                                              (dB)                                     __________________________________________________________________________    1      0.10  30    5500     96.67    25.0                                     2      0.11  30    5800     96.67    21.0                                     3      0.10  30    6300     105.00   10.0                                     4      0.11  30    6300     105.00   22.5                                     5      0.10  30    6800     113.33   27.0                                     6      0.11  30    6800     113.33   29.0                                     7      0.10  30    7300     121.67   25.5                                     8      0.11  30    7300     121.67   27.0                                     9      0.10  30    7800     130.00   25.5                                     10     0.11  30    7800     130.00   28.5                                     11     0.10  30    8300     138.33   25.5                                     12     0.11  30    8300     138.33   26.0                                     13     0.10  30    8800     146.67   22.5                                     14     0.11  30    8800     146.67   27.0                                     15     0.10  30    9300     155.00   25.0                                     16     0.11  30    9300     155.00   24.0                                     __________________________________________________________________________

                                      TABLE 4                                     __________________________________________________________________________    Specification of the                                                                            Specification of                                            impeller          the casing   Dominant level                                    Outside                                                                            Number                                                                             Blade                                                                              Tongue                                                                             Tongue  of tongue                                      Test                                                                             diameter                                                                           of   thickness                                                                          clearance                                                                          radius  interference                                                                          Casing                                                                            Impeller                           No.                                                                              (mm) blades                                                                             (mm) Cd (mm)                                                                            R (mm)                                                                             τ                                                                            sound Z (dB)                                                                          No. No.                                __________________________________________________________________________    1  99.0 10   0.5  2.7  2.0  0.28                                                                             35.0    3   20                                 2  99.0 30   2.0  2.7  2.0  0.47                                                                             30.0    3   23                                 3  99.0 60   0.5  2.7  2.0  0.58                                                                             24.3    3   6,11,21                            4  100.0                                                                              60   0.3  2.2  2.0  0.65                                                                             25.0    3   26                                 5  99.0 60   0.5  2.7  2.0  0.74                                                                             17.8    3   7,12                               6  99.0 60   1.0  2.7  2.0  0.73                                                                             15.0    3   22                                 7  100.0                                                                              80   0.3  2.2  2.0  0.78                                                                             17.0    3   27                                 8  99.0 80   0.5  2.7  2.0  0.90                                                                             8.9     3   8,13                               9  100.0                                                                              100  0.3  2.2  2.0  0.91                                                                             6.0     3   28                                 10 99.0 100  0.5  2.7  2.0  1.06                                                                             0.7     3   2,3,4,5                            11 99.0 120  0.3  2.7  2.0  1.23                                                                             0.0     3   16,25                              12 99.0 120  0.5  2.7  2.0  1.23                                                                             0.4     3   1,9,14,2,30                        13 99.0 144  0.3  2.7  2.0  1.42                                                                             1.0     3   17                                 14 99.0 144  0.5  2.7  2.0  1.44                                                                             0.0     3   10,15                              15 100.0                                                                              180  0.3  3.0  2.0  1.87                                                                             0.0     4   18                                 16 100.0                                                                              200  0.3  3.0  2.0  2.06                                                                             0.0     4   39                                 17 100.0                                                                              200  0.3  3.0  2.0  2.44                                                                             0.0     4   38                                 18 99.0 300  0.3  2.7  2.0  2.78                                                                             0.0     3   19                                 19 70.0 90   0.5  2.7  2.0  1.30                                                                             1.8     1   31,32                              20 70.0 100  0.3  2.7  2.0  1.40                                                                             0.0     1   35                                 21 70.0 120  0.3  2.7  2.0  1.64                                                                             0.0     1   36                                 22 150.0                                                                              200  0.5  2.6  2.0  1.29                                                                             0.0     8   33,34                              23 100.0                                                                              150  0.3  3.0  4.0  1.87                                                                             0.0     6   18                                 24 99.0 30   2.0  3.5  4.0  0.54                                                                             24.6    6   23                                 25 100.0                                                                              60   0.5  3.0  4.0  0.79                                                                             14.1    6   40                                 26 99.0 100  0.5  3.5  4.0  1.31                                                                             0.0     6   3                                  27 99.0 60   1.0  3.5  4.0  0.88                                                                             9.4     6   22                                 28 99.0 144  0.5  3.5  4.0  1.80                                                                             0.0     6   15                                 29 99.0 30   2.0  3.5  6.0  0.54                                                                             27.0    7   23                                 30 99.0 60   1.0  3.5  6.0  0.88                                                                             8.1     7   22                                 31 99.0 144  0.5  3.5  6.0  1.80                                                                             0.0     7   15                                 32 100.0                                                                              180  0.3  3.0  6.0  1.87                                                                             0.0     7   18                                 33 99.0 100  0.5  3.5  6.0  1.31                                                                             0.3     7   3                                  34 100.0                                                                              60   0.5  3.0  6.0  0.79                                                                             12.2    7   40                                 35 99.0 40   0.5  3.5  6.0  0.67                                                                             19.5    7   11                                 36 99.0 240  0.5  1.5  2.0  1.37                                                                             0.0     2   24                                 37 99.0 100  0.5  1.5  2.0  0.70                                                                             16.2    2   3                                  38 99.0 60   1.0  1.5  2.0  0.50                                                                             26.0    2   22                                 39 99.0 30   2.0  1.5  2.0  0.35                                                                             35.0    2   23                                 40 100.0                                                                              60   0.5  1.0  2.0  0.43                                                                             28.4    2                                      41 99.0 40   0.5  1.5  2.0  0.43                                                                             31.8    2   21                                 42 99.0 144  0.5  1.5  2.0  0.90                                                                             6.9     2   15                                 43 99.0 120  0.3  1.5  2.0  0.79                                                                             12.6    2   16                                 44 99.0 40   0.5  6.0  2.0  0.91                                                                             9.5     5   6,11,21                            45 99.0 60   1.0  6.0  2.0  1.35                                                                             0.0     5   22                                 46 99.0 144  0.5  6.0  2.0  2.92                                                                             0.0     5   15                                 47 99.0 30   2.0  6.0  2.0  0.73                                                                             14.7    5   23                                 __________________________________________________________________________

                  TABLE 5                                                         ______________________________________                                        (1)                                                                           Impeller (2)     (3)     (4)  (5)    (6)  (7)                                 No.      (Hz)    (dB)    (dB) (dB)   (dB) (dB)                                ______________________________________                                        11       4629.3  4.0     58.99                                                                              46.49  58.74                                                                              0.25                                23       2480.0  8.0     54.23                                                                              39.79  54.07                                                                              0.16                                21       3303.3  12.0    51.58                                                                              44.78  50.56                                                                              1.02                                11       3304.7  15.0    52.17                                                                              44.01  51.45                                                                              0.72                                23       3467.0  35.0    78.31                                                                              78.12  64.62                                                                              13.69                               23       2478.5  33.0    61.40                                                                              59.98  55.85                                                                              5.55                                22       6941.0  22.0    58.16                                                                              44.95  57.95                                                                              0.21                                21       3300.7  17.0    54.30                                                                              48.64  52.93                                                                              1.37                                3        11531.7 8.0     60.85                                                                              37.00  60.83                                                                              0.02                                3        8251.7  12.0    53.83                                                                              27.30  53.82                                                                              0.01                                12       4952.0  10.0    49.96                                                                              36.78  49.75                                                                              0.21                                23       2479.0  10.0    54.61                                                                              40.88  54.42                                                                              0.19                                23       2475.5  22.0    54.50                                                                              43.37  54.15                                                                              0.35                                15       11875.2 8.0     51.81                                                                              25.98  51.80                                                                              0.01                                23       3473.0  28.0    64.39                                                                              61.69  61.05                                                                              3.34                                15       7147.2  9.0     41.55                                                                              19.03  41.53                                                                              0.02                                15       8251.7  11.0    54.00                                                                              27.25  53.99                                                                              0.01                                11       4619.3  12.0    59.37                                                                              47.60  59.07                                                                              0.30                                23       3469.0  12.0    63.17                                                                              53.79  62.64                                                                              0.53                                23       1193.0  15.0    40.04                                                                              32.73  39.15                                                                              0.89                                12       4956.0  30.0    59.13                                                                              58.25  51.76                                                                              7.37                                6        4617.3  8.0     67.65                                                                              49.84  67.58                                                                              0.07                                15       11880.0 8.0     53.87                                                                              26.83  53.86                                                                              0.01                                21       4621.3  5.0     61.05                                                                              47.75  60.84                                                                              0.21                                15       5719.2  3.0     38.58                                                                              17.47  38.55                                                                              0.03                                15       7144.8  7.0     42.52                                                                              19.28  42.50                                                                              0.02                                ______________________________________                                         (2) Frequency of interference sound                                           (3) Dominant level of interference sound                                      (4) Aweighted, 1/3 octave bend overall sound level                            (5) 1/3 octave band sound level in the frequency of interference sound        (6) 1/3 octave band overall sound level without (5)                           (7) Difference between (4) and (6) ((4) - (6))                           

We claim:
 1. A method for making a multiblade radial fan comprising animpeller and a scroll type casing, comprising the step of forming theimpeller and the scroll type casing to satisfy the formula:

    θ.sub.z =tan.sup.-1 {0.295ε(1-nt/(2πr))(H/H.sub.t)ξ.sup.1.641 }

where 0.75≦ε≦1.25, n=a number of the radially directed blades, t=athickness of the radially directed blades, r=an outside radius of theimpeller, H=a height of the radially directed blades, H_(t) =a height ofthe scroll type casing, ξ=a diameter ratio of the impeller, θ_(z) =adivergence angle of the scroll type casing.
 2. A method for making amultiblade radial fan of claim 1, wherein the impeller and the scrolltype casing are formed to further satisfy the formula:

    3.0°≦θ.sub.z ≦8.0°.


3. A method for making a multiblade radial fan of claim 1, wherein theimpeller and the scroll type casing are formed to further satisfy theformula:

    0.4≦ξ≧0.8.


4. A method for making a multiblade radial fan of claim 1, wherein theimpeller and the scroll type casing are further formed to satisfy thecorrelation expressed by the formula:

    H/D.sub.1 ≦0.75

where D₁ =an inside diameter of the impeller.
 5. A method for making amultiblade radial fan of claim 1, wherein the impeller and the scrolltype casing are further formed to satisfy the formula:

    0.65≦H/H.sub.t.


6. A multiblade radial fan comprising an impeller and a scroll typecasing, wherein the impeller and the scroll type casing to satisfy theformula:

    θ.sub.z =tan.sup.-1 {0.295ε(1-nt/(2πr))(H/H.sub.t)ξ.sup.1.641 }

where 0.75≦ε≦1.25, n=a number of the radially directed blades, t=athickness of the radially directed blades, r=an outside radius of theimpeller, H=a height of the radially directed blades, H_(t) =a height ofthe scroll type casing, ξ=a diameter ratio of the impeller, θ_(z) =adivergence angle of the scroll type casing.
 7. A multiblade radial fanof claim 6, wherein the impeller and the scroll type casing furthersatisfy the formula:

    3.0°≦θ.sub.z ≦8.0°.


8. A multiblade radial fan of claim 6, wherein the impeller and thescroll type casing satisfy the formula:

    0.4≦ξ≦0.8.


9. A multiblade radial fan of claim 6, wherein the impeller and thescroll type casing the formula:

    H/D.sub.1 ≦0.75

where D₁ =an inside diameter of the impeller.
 10. A multiblade radialfan of claim 6, wherein the impeller and the scroll type casing satisfythe formula:

    0.65≦H/H.sub.t.


11. A method for making a multiblade centrifugal fan comprising animpeller having a plurality of blades circumferentially spaced from eachother and a scroll type casing accommodating the impeller, comprisingforming the scroll type casing such that a tongue located at or outsidea radial position where a ratio of a half band width of a jet flowdischarged from an interblade channel to a virtual interblade pitch is acertain value near
 1. 12. A method for making a multiblade centrifugalfan of claim 11, wherein an inter-blade pitch at a trailing edge of theblades is less than or equal to 5 mm and the number of blades is largerthan or equal to
 60. 13. A method for making a multiblade centrifugalfan comprising an impeller having a plurality of bladescircumferentially spaced from each other and a scroll type casingaccommodating the impeller, comprising forming the scroll type casingsuch that a tongue located at or outside a radial position where a ratioof a half band width of a jet flow discharged from an interblade channelto a virtual interblade pitch at a radial position where half bandwidths of two adjacent jet flows discharged from two adjacent interbladechannels are equal to a virtual interlade pitch is a certain valuenear
 1. 14. A method for making a multiblade centrifugal fan of claim13, wherein an inter-blade pitch at a trailing edge of the blades isless than or equal to 5 mm and the number of the blades is larger thanor equal to
 60. 15. A method for making a multiblade centrifugal fancomprising an impeller having a plurality of blades circumferentiallyspaced from each other and a scroll type casing accommodating theimpeller, wherein the impeller and the scroll type casing are formed tosatisfy the formula:

    -Aτ+B<10.0

where τ=b/δ₃, b=(δ₃ -c)(C_(d) /X)+c, c=Cδ₁, δ₁ ={(2πr)/n}-t, δ₃=2π(r+X)/n, C_(d) =a tongue clearance, n=a number of the blades, t=athickness of the blades, r=an outside radius of the impeller, and A, B,C, X=constants determined through tests.
 16. A method for making amultiblade centrifugal fan comprising an impeller having a plurality ofblades circumferentially spaced from each other and a scroll type casingaccommodating the impeller, wherein the impeller and the scroll typecasing are formed to satisfy the formula:

    -47.09τ+50.77<10.0

where τ=b/δ₃, b=(δ₃ -c)(_(d) /X)+c, X=0.8δ₂, c=0.3δ₁, δ₁ ={(2πr)/n}-t,δ₂ =2=(2πr)/n, δ₃ =2π(r+X)/n, C_(d) =a tongue clearance, n=a number ofthe blades, t=a thickness of the blades, r=an outside radius of theimpeller.
 17. A multiblade centrifugal fan comprising an impeller havinga plurality of blades circumferentially spaced from each other and ascroll type casing accommodating the impeller, wherein the scroll typecasing further comprises a tongue located at or outside the a radialposition where a ratio of a half band width of a jet flow dischargedfrom an interblade channel to a virtual interblade pitch is a certainvalue near
 1. 18. A multiblade centrifugal fan of claim 17, wherein aninter-blade pitch at a trailing edge of the blades is less than or equalto 5 mm and the number of the blades is larger than or equal to
 60. 19.A multiblade centrifugal fan comprising an impeller having a pluralityof blades circumferentially spaced from each other and a scroll typecasing accommodating the impeller, wherein the scroll type casingfurther comprises a tongue located at or outside a radial position wherea ratio of a half band width of a jet flow discharged from an interbladechannel to a virtual interblade pitch at a radial position where halfband widths of two adjacent jet flows discharged from two adjacentinterblade channels are equal to a virtual interblade pitch is a certainvalue near
 1. 20. A multiblade centrifugal fan of claim 19, wherein aninter-blade pitch at a trailing edge of the blades is less than or equalto 5 mm and the number of the blades is larger than or equal to
 60. 21.A multiblade centrifugal fan comprising an impeller having a pluralityof blades circumferentially spaced from each other and a scroll typecasing accommodating the impeller, wherein the impeller and the scrolltype casing satisfy the formula:

    -Aτ+B<10.0

where τ=b/δ₃, b=(δ₃ -c)(C_(d) /X)+c, c=Cδ₁, δ₁ ={(2πr)/n}-t, δ₃=2π(r+X)/n, C_(d) =a tongue clearance, n=a number of the blades, t=athickness of the blades, r=an outside radius of the impeller, and A, B,C, X=constants determined through tests.
 22. A multiblade centrifugalfan comprising an impeller having a plurality of bladescircumferentially spaced from each other and a scroll type casingaccommodating the impeller, wherein the impeller and the scroll typecasing satisfy the formula:

    -47.09τ+50.77<10.0

where τ=b/δ₃, b=(δ₃ -c)(_(d) /X)+c, X=0.8δ₂, c=0.3δ₁, δ₁ ={(2πr)/n}-t,δ₂ =(2πr)/n, δ₃ =2π(r+X)/n, C_(d) =a tongue clearance, n=a number of theblades, t=a thickness of the blades, r=an outside radius of theimpeller.
 23. A method for driving an impeller of a multiblade radialfan, comprising the step of driving the impeller so as to make a flowcoefficient φ equal to

    0.295ε(1-nt/(2πr))ξ.sup.1.641

where 0.75≦ε≦1.25, n=a number of the radially directed blades, t=athickness of the radially directed blades, r=an outside radius of theimpeller, ξ=a diameter ratio of the impeller.
 24. A method for drivingthe impeller of a multiblade radial fan of claim 23, wherein ξ is in therange of

    0.4<ξ<0.8.


25. A method for making a multiblade centrifugal fan comprising animpeller having a plurality of blades circumferentially spaced from eachother and a scroll type casing accommodating the impeller, wherein theimpeller and the scroll type casing are formed to satisfy the formula:

    {(δ.sub.3 -c)(C.sub.d /X)+c}/δ.sub.3 ≧1

where c=Cδ₁, δ₁ ={(2πr)/n}-t, δ₃ =2π(r+X)/n, C_(d) =a tongue clearance,n=a number of the blades, t=a thickness of the blades, r=an outsideradius of the impeller, and C, X=constants determined through tests. 26.A method for making a multiblade centrifugal fan of claim 25, wherein aninter-blade pitch at a trailing edge of the blades is less than or equalto 5 mm and the number of blades is larger than or equal to
 60. 27. Amethod for making a multiblade centrifugal fan comprising an impellerhaving a plurality of blades circumferentially spaced from each otherand a scroll type casing accommodating the impeller, wherein theimpeller and the scroll type casing are formed to satisfy the formula:

    {(δ.sub.3 -c)(C.sub.d /X)+c}/δ.sub.3 ≧0.87

where X=0.8δ₂, c=0.3δ₁, δ₁ ={(2πr)/n}-t, δ₂ =(2πr)/n, δ₃ =2π(r+X)/n,C_(d) =a tongue clearance, n=a number of the blades, t=a thickness ofthe blades, r=an outside radius of the impeller.
 28. A method for makinga multiblade centrifugal fan of claim 27, wherein an inter blade pitchat a trailing edge of the blades is less than or equal to 5 mm and thenumber of the blades is larger than or equal to
 60. 29. A multibladecentrifugal fan comprising an impeller having a plurality of bladescircumferentially spaced from each other and a scroll type casingaccommodating the impeller, wherein the impeller and the scroll typecasing satisfy the formula:

    {(δ.sub.3 -c)(C.sub.d /X)+c}/δ.sub.3 ≧1

where c=Cδ₁, δ₁ ={(2πr)/n}-t, δ₃ =2π(r+X)/n, C_(d) =a tongue clearance,n=a number of the blades, t=a thickness of the blades, r=an outsideradius of the impeller, and C, X=.
 30. A multiblade centrifugal fan ofclaim 29, wherein an inter-blade pitch at a trailing edge of the bladesis less than or equal to 5 mm and the number of the blades is largerthan or equal to
 60. 31. A multiblade centrifugal fan comprising animpeller having a plurality of blades circumferentially spaced from eachother and a scroll type casing accommodating the impeller, wherein theimpeller and the scroll type casing satisfy the formula:

    {(δ.sub.3 -c)(C.sub.d /X)+c}/δ.sub.3 ≧0.87

where X=0.8δ₂, c=0.3δ₁, δ₁ ={(2πr)/n}-t, δ₂ =(2πr)/n, δ₃ =2π(r+X)/n,C_(d) =a tongue clearance, n=a number of the blades, t=a thickness ofthe blades, r=an outside radius of the impeller.
 32. A multibladecentrifugal fan of claim 31, wherein an inter-blade pitch at a trailingedge of the blades is less than or equal to 5 mm and the number of theblades is larger than or equal to 60.